A Coanda flow amplifier of the above type is disclosed, for example, in U.S. Pat. No. 5,974,802. The operating principle of a Coanda flow amplifier is based on the phenomenon known as the Coanda effect (i.e., a fluid flowing along a curved surface tends to follow the outline of the curved surface). Consequently, a Coanda flow amplifier usually includes an inlet for a fluid flow to be amplified, a fluid channel, which is bordered by a curved surface and which—along the flow direction of the flow to be amplified—at first narrows and subsequently widens with a funnel shape, and a fluid outlet. A drive-flow inlet is provided, radially with respect to the fluid channel, in an area upstream of the constriction of the fluid channel. The inlet is fluid-connected with the fluid channel via a drive-flow discharge slit.
The drive-flow inlet serves to supply the Coanda flow amplifier with a drive fluid at a given intake pressure that subsequently reaches a high flow velocity (typically sonic velocity) when passing through the drive-fluid discharge slit, and subsequently flows through the fluid channel along the surface that borders the fluid channel. This generates suction in the area of the intake of the Coanda flow amplifier, as a result of which large volumes of the fluid to be conveyed by the Coanda flow amplifier are sucked into the intake.
As described in U.S. Pat. No. 5,974,802, a Coanda flow amplifier of this type can be arranged in the exhaust gas recirculation line of an internal combustion engine, to recirculate exhaust gases produced by the internal combustion engine.
German patent document DE 100 01 717 C1 describes the use of a Coanda flow amplifier in a fuel cell system, which comprises a fuel cell unit, a cathode gas supply line connected to the cathode side of the fuel cell unit, a cathode exhaust gas return line for the recirculation of cathode exhaust gas that is also connected to the cathode side of the fuel cell unit, as well as an anode exhaust gas return line that serves to recirculate the anode exhaust gas and is connected to the anode side of the fuel cell unit. The Coanda flow amplifier may be arranged in the cathode gas supply line and/or in the cathode exhaust gas return line of the fuel cell system, whereby a drive-flow inlet of the Coanda flow amplifier is connected to a compressed-air source via a compressed-air line. Alternatively, the Coanda flow amplifier may be arranged in the anode exhaust gas return line. In this case, the drive-flow inlet of the Coanda flow amplifier is connected to a fuel gas pressure tank, which may for example contain gaseous or liquid hydrogen.
As explained above, for proper operation of the Coanda flow amplifier it is necessary that the drive fluid, during its passage through the drive-flow discharge slit, be accelerated to a very high flow velocity, typically the velocity of sound. This can be ensured if a pressure ratio between the discharge pressure of the drive flow when it leaves the drive-flow discharge slit, and an intake pressure of the drive flow when it enters into the drive-flow discharge slit, does not exceed a critical pressure ratio set in dependence on the desired flow velocity of the drive fluid when it leaves the drive-fluid discharge slit. For an acceleration of the drive-fluid flow to sonic velocity (Mach 1) and diatomic gases (Kappa=1.4) the critical pressure ratio is calculated as 0.528. To prevent the critical pressure ratio from being exceeded (i.e., to ensure a proper functioning of the Coanda flow amplifier), the drive fluid is usually supplied to the Coanda flow amplifier at a sufficiently high supply pressure, which can be pre-set by means of a pressure controller.
However, in some applications, and particularly for the use of a Coanda flow amplifier in a fuel cell system, the problem arises that the mass flow of the drive fluid to be supplied to the Coanda flow amplifier, and thus the drive fluid's supply pressure, will also be affected by other system parameters. For example, if the Coanda flow amplifier is to be used in a fuel cell system to recirculate the anode exhaust gas, and if the fuel gas to be supplied to the anode side of the fuel cell is to be used as drive fluid, then the fuel gas volume to be supplied to the fuel cell depends on the fuel gas consumption in the fuel cell (i.e., on the load state of the fuel cell). Thus, under low-load conditions of the fuel cell the pre-set intake pressure of the drive fluid may not be sufficient to accelerate the drive-fluid flow to a sufficiently high velocity when it passes through the drive-fluid discharge slit, so that the proper functioning of the Coanda flow amplifier can no longer be ensured.